Lymph node and peripheral blood lymphocytes in a case of Hodgkin's disease (mixed cellularity) were studied using May-Grünwald-Giemsa (MGG), acid naphthyl acetate esterase (ANAE), immunoperoxidase staining and lymphocyte surface markers, autoradiographic, and lymphocyte stimulation techniques. According to MGG staining and autoradiographic studies of lymph, node cells, small lymphocytes, intermediate lymphoid cells, and large mononuclear cells resembling in-vitro stimulated immunoblasts, Hodgkin's cells and Reed-Sternberg (RS) cells formed a morphologically continuous DNA synthetizing series. A large majority of small lymphocytes from a lymph node were ANAE positive, thus being T-lymphocytes, and formed rosetts around large mononuclear cells and RS cells. Most RS and large mononuclear cells had ANAE positive spots in the cytoplasm, thus resembling T-lymphocytes more than diffusely staining monocytes. These cells did not contain cytoplasmic immunoglobulin and were muramidase negative. Both lymph node and peripheral blood lymphocytes responded strongly to PHA. The role of T-lymphocytes in Hodgkin's disease and the origin of RS cells are are discussed on the basis of the findings.